1. Field of the Invention
The present invention relates to a thick film glaze resistor. More particularly the present invention relates to a resistor composition which can be fired in a non-oxidizing atmosphere such as a neutral atmosphere or a reducing atmosphere to form a resistor on a copper thick film hybrid integrated circuit (Cu-HIC) substrate or the like with the resistor coexisting with a base metal electrode, particularly a copper electrode, a resistor produced therefrom, and a method of producing a resistor.
2. Description of Prior Art
Demands for miniaturization and multi-functionalization of apparatuses have recently been growing year after year. To meet these demands, integration of circuits, and high-density packaging of circuit parts have become important techniques. Accordingly, passive elements such as capacitors and resistors have a tendency to be made in the form of thick film elements from the viewpoint of ease in packaging thereof on circuit substrates as well as miniaturization thereof.
Among conventional thick film elements, thick film resistors have heretofore been produced by using ruthenium oxide as a conductive phase and a lead borosilicate glass as an inorganic binder for fixing the resistor onto a ceramic substrate. The ruthenium oxide resistor is formed by a conventional thick film process, the basic procedure of which comprises screen printing, drying and firing.
The thick film process will be briefly described. Screen printing is effected by squeezing out a thick film composition in the form of a paste through an open pattern of a screen, which has been prepared by coating a stainless steel mesh with a resin resist and removing the resin resist only from part thereof to constitute the necessary open pattern, onto a substrate by means of a squeegee. By doing so, a necessary pattern of the composition corresponding to that of the stainless screen is formed on the substrate. After the printing, the film on the substrate is dried at 100.degree. to 150.degree. C. to remove the solvent contained in the film of the thick film paste through evaporation thereof. Thereafter, the film is fired generally in the air at a peak temperature of 600.degree. to 1000.degree. C. In this firing step, an organic polymer contained in the thick film composition for the purpose of providing printability therefor is decomposed by oxidation thereof with temperature elevation. Thereafter, the glass serving as an inorganic binder is softened and melted, and the melted glass solidifies again in the course from the peak temperature to ordinary temperatures, whereupon the composition, when it is a thick film resistor composition, adheres to the substrate with the conductive phase held in the glass matrix. Such thick film technique is discussed in detail by Planer, Philips in "Thick Film Circuit", LONDON BUTTERWORTH.
In the case where a ruthenium oxide resistor material is used, however, since the firing is effected in air, a noble metal material such as silver and palladium must be used as an electrode. Thus, conventional thick film systems using a ruthenium oxide resistor material is very expensive because a noble metal must be used therein as a conductor composition and a resistor composition. Further, various measures such as covering with a protective film must be taken to prevent wasting silver in soldering or from migrating.
Even if thought is given to formation in air of an RuO.sub.2 -glass type glaze resistor on a base metal electrode of, for example, tungsten, molybdenum or copper, the electrode material undergoes oxidation to make it impossible to form the glaze resistor on the electrode. In view of this, in order to form a glaze resistor on a base metal electrode, the glaze resistor composition must be fired in a reducing atmosphere or a neutral atmosphere. In this case, however, an RnO.sub.2 glaze resistor material is defective in that the ruthenium oxide is reduced in the nature of the case to metallic ruthenium, when the glaze resistor material is fired in a non-oxidizing atmosphere, thereby failing to secure any characteristics of a resistor. In other words, the coexistence of a ruthenium oxide thick film resistor composition with a base metal electrode material such as copper was very difficult.
Thick film resistor compositions which can coexist with a base metal electrode material such as a copper electrode include a thick film resistor composition disclosed in U.S. Pat. No. 4,039,997 which comprises molybdenum silicide, tungsten silicide, etc. as a conductive phase, and a barium borosilicate glass as a glass phase. In the case of this thick film resistor composition, however, since the peak firing temperature is as high as 970.degree. to 1,150.degree. C., the life span of the furnace is unfavorably shortened in actual production. Further, since the peak temperature for thick film conductor materials for copper electrodes which are generally commercially available today is 900.degree. C., the production of resistors from the above-mentioned thick film resistor composition by firing requires two types of furnaces with different peak temperatures or a furnace in which the peak temperature must be changed. Therefore, there arises such a problem that an excessive investment may be involved, or the production efficiency cannot be raised. Furthermore, since the particle diameter of the glass powder used in the thick film resistor composition is as small as 1 to 2 .mu.m, the glass powder surfaces are melted before escape of an organic polymer contained in the thick film resistor composition through thermal decomposition thereof when the composition is fired in a non-oxidizing atmosphere, with the result that the organic polymer is retained in the form of carbon in the resulting resistor. This results in disadvantages such as destabilization of the thermal coefficient of the resistor and instability of the humidity resistance characteristics of the resistor. Further, where the particle diameter of the silicide in the thick film resistor composition exceeds 1 .mu.m, the radius of the silicide particles is too large for the glass, with the result that wettability between the glass particles and the silicide particles becomes so poor that many voids are formed in the resulting sintered resistor. As a result, the conductor material to be connected to the resistor will diffuse into the thick film resistor due to a thermal diffusion phenomenon thereof in the course of firing of the thick film resistor composition, resulting in such a disadvantage that the sheet resistivity of the resulting sintered resistor is unstable.
U.S. Pat. No. 4,119,573 (Ishida et al.) discloses a thick film resistor composition comprising molybdenum silicide, magnesium silicide, tantalum silicide, and manganese silicide as conductive phase, and, as an inorganic binder, barium borosilicate glass containing 0 to 7 wt. % of niobium pentoxide. However, it is difficult to secure desired resistance characteristics by dispersing the above-mentioned thick film resistor composition in a vehicle containing ethyl cellulose dissolved therein, forming a film by the screen printing method, and firing the film on a ceramic substrate in a non-oxidizing atmosphere as described in the above-mentioned patent. This is so because ethyl cellulose is carbonized due to its thermal oxidation decomposability, when exposed to high temperatures in a non-oxidizing atmosphere having a very low oxygen concentration, to remain in the form of residual carbon in the resulting sintered resistor to deteriorate the resistance characteristics of the resistor. When a thick film resistor composition (glass silicide ) of Ishida et al. is screen-printed using a vehicle containing a heat decomposable organic polymer used by us and is fired in a non-oxidizing atmosphere, the resistance is too scattered, and therefore high uncertainty remains in its practical use. Particularly, with the same area, the sheet resistivity varies depending on the length/width ratio (aspect ratio) of the resistor film, leading to a difficulty in designing a resistance value.
As against the above-mentioned resistor composition comprising silicides as a conductor phase, a thick film resistor composition comprising a boride such as lanthanum hexaboride as a conductor phase is disclosed in U.S. Pat. No. 4,512,917. However, this thick film resistor composition has a defect that deterioration of the resistance characteristics of the resistor formed therefrom occurs particularly in a high resistance region where a relatively large amount of the glass phase is present.
Donohue et al., "Nitrogen-Fireable Resistors: Emerging Technology For Thick Film Hybrids" Proceedings of 1987 Electronic Components Conference, May, 1987 disclosed a technique of forming fine tantalum boride by reducing tantalum pentoxide and boron oxide with the strongly reducing agent lanthanum hexaboride. According to this method, however, lanthanum trioxide is formed as a by-product and is in the nature of being slightly soluble in cold water. The resistor obtained after sintering may be in a state entirely covered with lanthanum trioxide formed by the disclosed reaction when the resistor is viewed in its entirety with consideration given to the particle diameter ratio of the lanthanum hexaboride particles to the glass particles. Since the whole of the resistor is covered with lanthanum trioxide in the nature of being soluble in cold water, it is believed that the conductor particles are flowed out together with the glass in an environmental test under high-temperature and high-humidity conditions. Because of this phenomenon, the resistor of Donohue et al. requires a glass coat without fail in order to attain a fluctuation of resistance value of less than 1%. This is a serious disadvantage because other steps of printing, drying, and firing must be taken for the formation of a thick film hybrid IC, which lengthens the production process and results in a higher cost. Further, this will become a hindrance to conversion from a noble metal type system for firing in the air to a base metal type copper thick film system in view of the fact that conventional ruthenium oxide resistors require no such glass coat.
In addition, in the case bf boride-glass type glaze resistance materials, it is very difficult to obtain a sheet resistivity of 10 k.OMEGA./.quadrature. or over because the thermal coefficient of resistor (TCR) is a large negative value (-300 ppm/.degree.C. or lower). Accordingly, the present state is that a conductor material such as tin oxide is used to obtain a higher sheet resistivity, particularly a sheet resistivity of 100 k.OMEGA./.quadrature. or higher. Therefore, paste blending of a resistor paste for 10 k.OMEGA./.quadrature. with a resistor paste for 100 k.OMEGA./.quadrature. is impossible. Further, simultaneous firing of a boride type resistance material and a tin type resistance material was difficult due to a mutual reaction therebetween.
A resistor must have good anti-surge characteristics since it is a circuit part. In this point, the glaze resistor produced from the boride-glass type glaze resistor material undergoes a conspicuous change in the resistance value by a surge voltage in view of a difficulty in providing a particle diameter of 1 .mu.m or smaller for the boride powder constituting the boride glass type glaze resistor material which difficulty is attributed to the preparation of the boride powder by preliminary synthesis and mechanical grinding, and because a non-uniform electric field distribution is developed inside the glass resistor.
A silicide type resistor disclosed in U.S. Pat. No. 4,695,504 that was developed by us is a resistor having very excellent humidity resistance and aspect dependency but with a limitation of 10 .OMEGA./.quadrature. to 10 k.OMEGA./.quadrature. in the region wherein characteristics of the resistor are satisfied. This is so because a resistor of 10 k.OMEGA./.quadrature. or higher not only shows a high negative value of thermal coefficient, of resistor but also has a high dependency of the resistance value on firing peak temperature, leading to imposition of a large limitation on the resistor forming process.